Vehicle fuel systems are required to control emission of fuel vapor. This is done by collecting vapor emitted from the fuel tank in a purge canister containing carbon to absorb the vapor. The canister is purged of collected vapor when the engine is running by drawing air through the canister into the engine, relying on manifold vacuum. The system is sealed except for venting to the atmosphere via the purge canister. On-board vapor integrity testing is required to a warning is given if vapor loss from the sealed system exceeds predetermined levels. Typical known vapor integrity testing systems are described U.S. Pat. Nos. 5,333,590 and 5,765,121.
The latter patent describes a basic test in which the manifold vacuum is used to pump out the fuel tank and the return of tank pressure to atmospheric ("bleedup") is monitored. If bleedup exceeds a certain threshold value R the system is determined to have an unacceptable vapor integrity. If the bleedup is less than R, it assumed that vapor integrity is acceptable. Low level loss of vapor integrity cannot be reliably detected with this basic system because vapor generation from fuel in the tank can cause pressure in the evacuated system to recover more rapidly than air ingress due to a low level loss of vapor integrity.
In addition, the bleedup for a particular level of vapor integrity depends on vapor volume, that is the volume of free space above the fuel tank and in the purge canister and connecting passages. Vapor volume is itself directly related to fuel level.
Thus, in order to improve the sensitivity of the basic bleedup test, measures must be taken to correct for different operating conditions, particularly the fuel level and the rate of vapor generation in the tank.
For example, U.S. Pat. No. 5,333,590 uses a threshold value R which is not fixed but is related to vapor volume and fuel temperature.
It is also known to improve the sensitivity of vapor integrity testing by using a two stage test. The first stage is a bleedup test in which pressure increase over a certain period (period_A) is measured. A second stage is carried out in which pressure rise of the closed system from atmospheric over a second period (period_B) is monitored. The second stage gives an indication of vapor generation in the tank under prevailing conditions. A constant scaling factor is used to deduct a proportion of pressure rise found during the second stage to provide a value which more closely represents the level of bleedup due to air ingress into the tank during the first stage of the test.
A source of error that is not dealt with in the existing systems described above arises from variations in temperature of the gaseous contents of the tank at the start of bleedup, due in the main to variations in the evacuation. Evacuation results in the temperature of the vapor contents being reduced below ambient temperature by an amount which depends on the nature of the evacuation (fast, slow, early or late). Without any compensation for such temperature variation, a worst case error in may be equivalent to a hole diameter of around 0.5 mm. Errors of this magnitude are not acceptable when small leaks equivalent to 0.5 mm diameter hole are required to be detected.